"We know that manic depression, or bipolar disorder, is a highly heritable disorder," says Miller. "But the world is having a lot of trouble trying to find genes [involved in] bipolar disorder."

He says one of the key problems is that the diagnosis of people with manic depression can be unreliable.

Miller and colleagues have been trying to establish a biological marker for manic depression, which could be used to complement clinical diagnosis.

To do this, they have been studying an artificially-induced visual phenomenon known as 'binocular rivalry'.

In everyday life, the left and right eyes are looking at more or less the same thing and your brain combines both inputs, allowing you to perceive distances," says Miller.

But if each eye is given a completely different image to look at, it induces an abnormal state.

Rather than combine the images, the brain first focuses on one image and then flips to the other.

"It's a perceptual flip phenomenon," says Miller.

He says the flip usually occurs on average every 1 to 2 seconds but this rate is known to vary widely between individuals.

Studying flip rates

In two previous studies Miller, working with Professor Jack Pettigrew at the University of Queensland, tested the flip rate in more than 200 subjects given special goggles that showed horizontal stripes to one eye and vertical stripes to the other.

They found that people with manic depression had a statistically significantly slower average flip rate - of around 3 to 4 seconds, and some were as slow as 7 to 10 seconds.

But to establish whether this difference in flip rate was a true biological marker for manic depression and not, for example, a side-effect of medication, further research was needed.

Identical twins had a more similar flip rate than non-identical twins, which was evidence that flip rate is inherited, says Miller.

"If the difference in flip rate was due to a side-effect of medication, you would not expect this result," he says.

Miller says genetics is responsible for 52% of the variability in flip rate and suggests it is a biological marker of the inherited bipolar disorder.

"That finding has clinical relevance," he says.

Diagnostic test?

But Miller says it's too early to say whether measuring the flip rate could be used as a diagnostic test for bipolar disorder.

He says the flip rate needs to be verified, for example, in large numbers of people with established clinical diagnoses of manic depression.

Studies in asymptomatic relatives of people with manic depression may also show that the flip rate is a useful biological marker for predisposition to the condition, says Miller.

And he says using the flip rate, instead of clinical diagnosis, to categorise people in genetic studies on mental illness could help net more of the genes involved.

He emphasises such studies would not lead to definitive genetic tests for manic depression and other mental illnesses, but could identify genes that help make people susceptible to such conditions.

Miller says it is possible that the flip rate may prove to be a biological marker for susceptibility to both schizophrenia and bipolar disorder, which can be hard to distinguish using clinical diagnoses.

Psychiatrist Professor Gordon Parker of Sydney's Black Dog Institute says there is much more work required to establish the flip rate as a marker but agrees the development of biological markers are important.

"The field (for both depressive and bipolar disorders) would be distinctly advanced by such markers as there have been no definitive ones up to now," says Parker.